Bend Tolerant Dual-Core Fiber and Cable for Balanced Photonic Links

ABSTRACT

A multicore fiber optic cable comprising of a dual core optical fiber having a dual core optical fiber geometry, the cores are spiraled parallel to one another along the longitudinal axis of the fiber to negate link path length difference, a coating that surrounds the fiber, a buffer tube that surrounds the coated fiber, a strength member that surrounds the buffer tube, and an outer jacket that surrounds the strength member.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout payment of any royalties thereon or therefor.

BACKGROUND

Fiber optic based photonic links are often used in high bandwidth analogand digital communication applications. Photonics and fiber opticsoffers numerous advantages over traditional radiofrequency hardware andelectrical interconnects for various analog link applications. Suchadvantages include reduced weight, immunity to electromagneticinterference, increased flexibility, larger bandwidth in fiber, andreduced loss in fiber. Many analog photonic link applications havecritical sensitivity and linearity requirements. Noise figure, linkgain, compression dynamic range and spurious free dynamic range are keyperformance parameters for analog links. Intensity modulation withdirect detection (IMDD) is an analog link modulation scheme where theintensity of an optical source is modulated by the analog signal.Demodulation is achieved through direct detection of the optical carrierand conversion using a photodetector. FIG. 1 (not admitted to be priorart) from Naval Research Laboratory report NRL/MR/5652-07-9065(incorporated by reference and not admitted to be prior art) allows oneto understand the schematic and operation of a traditional unbalancedIMDD analog photonic link 20. Polarization-maintaining optical fiber 21connects the laser 22 to the Mach-Zehnder modulator (MZM) 24. Asingle-core single-mode optical fiber 23 connects the MZM 24 to a singlephotodetector 25. RF i/p=RF input to the MZM 26. RF o/p=RF out 28 fromthe photodetector. A bias control circuit 27 sets the MZM operatingpoint at quadrature.

Balanced IMDD links offer a performance advantage over traditionalunbalanced IMDD links in terms of noise suppression and link gain (seereferences 1-3 in J. Diehl, et al, “Measurement and discussion of abalanced photonic link utilizing dual-core optical fiber,” Proc. IEEEAvionics and Vehicle Fiber Optics and Photonics Conference, 2019, thisreference is not admitted to be prior art). FIG. 2 (not admitted to beprior art) 30 illustrates a balanced IMDD link schematic based on twoindividual single-core fibers 31 interfaced to the output of adual-output MZM 32 and to the input of a balanced photodetector 33.Using both output arms of the dual-output MZM results in twice thephotocurrent collected. This corresponds to four times (6 dB) more gain.More importantly, common mode noise (such as laser noise or spontaneousemission noise arising from a pre-modulator erbium-doped fiberamplifier) is differenced at the balanced photodetector, resulting insignificant noise suppression in a typical link. Efficient use ofoptical power and noise cancellation is achieved at the same timeresulting in higher link gain, lower noise figure, and higher spuriousfree dynamic range.

Building a balanced link becomes progressively more difficult as themodulation frequency increases. This is due to the ever-tighteningphase-tolerance as the modulation signal's frequency increases andwavelength decreases. At frequencies above 10 GHz, maintaining steadybalanced phase over any appreciable transmission distance (severalmeters) is limited by temperature and physical effects on the twoindividual single-core optical fibers interfaced to the output of adual-output Mach-Zehnder modulator (MZM) and to the input of a balancedphotodetector. Again referring to FIG. 2, any effective change in fiberoptic length on the order of micrometers can begin to unbalance a highfrequency analog photonic link.

A balanced IMDD analog photonic link based on one dual-core opticalfiber can mitigate the temperature and physical effects that cause ahigh frequency analog photonic link to become unbalanced over anyappreciable link distance (see FIG. 3, not admitted to be prior art) 40.With both outputs of the modulator traveling down independentsingle-mode cores in a single fiber 45, the effects of temperature andmechanical stress is minimized wherein any effect impacting one corewould be expected to impact the other core as well.

Fiber optic cable bending is one form of mechanical stress, so thereforefiber optic cable bending can cause the link to become unbalanced. Asshown in FIG. 8, in a standard, dual-core optical fiber with the cores61 traveling straight down the fiber, a net path length difference canbe seen over long bend distances. As shown in FIG. 9, by rotating thecores 63 around a central axis 64, the net effect of this path lengthdifference accumulation is limited to the modulus of the spiral lengthand the total bend length (that is to say the total path lengthdifference cannot be greater than the spiral length 62). The dual-corefiber optic cable design described in this disclosure is meant tominimize differences in path delay due to fiber optic cable bending,thereby mitigating the net effect of bending the cable in any directionwhen the fiber optic cable is installed on an aerospace platform orother physical structure.

SUMMARY

The present invention is directed to a balanced photonic link with theneeds enumerated above and below.

The present invention is directed to a balanced photonic link based ondual core optical fiber with spiraled cores.

It is a feature of the present invention to provide a balanced photoniclink design based on dual core optical fiber with spiraled cores.

It is a feature of the present invention to provide a balanced photoniclink design based on a fiber optic cable that has a dual core opticalfiber with spiraled cores.

It is a feature of the present invention to provide a cable design witha buffer tube, strength member and outer jacket.

DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims, and accompanying drawings wherein:

FIG. 1 is a schematic of a traditional intensity modulation with directdetection photonic link based on one single core optical fiber and onephotodetector;

FIG. 2 illustrates a balanced IMDD analog photonic link with two singlecore optical fibers and a balanced photodetector;

FIG. 3 illustrates a balanced IMDD analog photonic link with one dualcore optical fiber and a balanced photodetector;

FIG. 4 illustrates a longitudinal cross-section view a fiber optic cablewith dual core optical fiber with spiraled cores;

FIG. 5 illustrates a transverse cross-section view of a dual core fiberoptic cable;

FIG. 6 illustrates a longitudinal cross section view of the dual corefiber with spiraled cores; and,

FIG. 7 illustrates a balanced IMDD analog photonic link utilizing a dualcore optical fiber with spiraled cores. Dual core fiber optic connectorswith optical fan-in/fan-out fibers or waveguides couple light into andout of the dual core optical fiber cable.

FIG. 8 shows that for two conventional cores inside of a bent opticalfiber, a net accumulation of path length is obtained (the slightdifference in the bend radius of each core results in a different arclength).

FIG. 9 shows that for two cores rotating around a central axis, thiseffect is averaged out, limiting the net path length difference to, atmost, one spiral length.

DESCRIPTION

The preferred embodiments of the present invention are illustrated byway of example below and in FIGS. 4-7. As seen in FIG. 4, a multicorefiber optic cable 10 includes a dual core fiber 100, a fiber coating200, a buffer tube 300, a strength member 400, and an outer jacket 500.The dual core fiber 100 has a fiber outer surface 105. The fiber coating200 surrounds the fiber outer surface 105 of the dual core fiber 100.The dual core optical fiber 100 has a spiraled dual core optical fibercore geometry 110. The dual core fiber cores are surrounded by acladding 120. The dual core optical fiber 100 cores are spiraled alongthe longitudinal axis to negate link path length difference. The buffer300 surrounds the fiber coating 200 and the dual core optical fiber 100.The strength member 400 and outer jacket 500 surrounds the buffer tube300. FIG. 7 illustrates a balanced IMDD analog photonic link 50utilizing multicore fiber optic cable 10 with a dual core fiber 100 withspiraled dual core optical fiber core geometry. Dual core fiber opticconnectors 38 with optical fan in 35/fan out 36 fibers or waveguidescouple light in to and out of the dual core optical fiber cable 10 witha dual core fiber 100 and spiraled dual core optical fiber core geometry110.

In the description of the present invention, the invention will bediscussed in a military environment; however, this invention can beutilized for any type of application that requires use of fiber opticcable.

The dual core fiber 100 may be, but without limitation, a plastic fiber,glass fiber, or any material practicable. The multicore fiber opticcable 10 may further include a strength member 400 that is disposedwithin cable.

The buffer tube 300 can be made from polymer, while strength member 400may be manufactured from fiberglass, Kevlar or any other materialpracticable. The cable 10 may further include an outer jacket 500 on theoutside of all the other elements. The outer jacket 500 may bemanufactured from polymer or any other material practicable.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a,” “an,” “the,” and “said” areintended to mean there are one or more of the elements. The terms“comprising,” “including,” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

Although the present invention has been described in considerable detailwith reference to certain preferred embodiments thereof, otherembodiments are possible. Therefore, the spirit and scope of theappended claims should not be limited to the description of thepreferred embodiment(s) contained herein.

What is claimed is:
 1. A multicore fiber optic cable comprising of; adual core optical fiber having a dual core optical fiber geometry, thecores are spiraled parallel to one another along the longitudinal axisof the fiber to negate link path length difference; a coating thatsurrounds the fiber; a buffer tube that surrounds the coated fiber; astrength member that surrounds the buffer tube; and an outer jacket thatsurrounds the strength member.
 2. The multicore fiber optic cable ofclaim 1, wherein the cable is employed in a balanced photonic link.